The present invention relates to an electromagnetic induction type cooker, and more particularly to a heating apparatus for an electromagnetic cooker having the improved electromagnetic induction characteristic and also manufacturing method thereof.
A general electromagnetic induction type cooker generates an eddy current in a metallic container which holds food to be cooked and cooks the food by heating the metallic container by the generated eddy current. As shown in FIG. 1, the conventional electromagnetic cooker comprises a heating apparatus 10 having a working coil 11 and ferrite core 12. One end of the working coil 11 is connected to the collector of a transistor Q.sub.1. The emitter of the transistor Q.sub.1 is connected to a grounded potential GND. The transistor Q.sub.1 is turned on/off by a pulse signal supplied from a comparator or an oscillator to its base, thereby opening/closing the current passage of the working coil 1. The pulse signal has a high frequency over 20 KHz . At this time, the working coil 11 generates an AC magnetic field having a high frequency and supplies it to a container 10 disposed on the ceramic plate 15. An eddy current is generated by the AC magnetic field of the high frequency in the container, which then is heated by the eddy current. The electromagnetic cooker additionally comprises a capacitor C.sub.2 connected to both ends of the working coil 11, a diode D.sub.1 connected between one end of the working coil 11 and the grounded potential GND, and a capacitor C connected between the other end of the working coil 11 and the grounded potential GND. The diode D.sub.1 functions to protect the transistor Q.sub.1 and the capacitor C.sub.2 functions to remove the impulse noise generated when the transistor Q.sub.1 is switched. The capacitor C.sub.2 determines the frequency of pulse signal which will be supplied to the base of the transistor Q.sub.1, when a comparator is connected to the base of transistor Q.sub.1 . At this time, the comparator compares. the both-end voltages of the working coil 11 and supplies the comparing signal to the base of transistor Q.sub.1 as a switching pulse signal.
FIGS. 2 and 3 are a plan view and cross-sectional view of the heating apparatus 10 shown in FIG. 1. Referring to FIGS. 2 and 3, the heating apparatus 10 comprises a mounting plate 13 on whose upper surface six ferrite cores 12 are installed. A passing hole 13-1 is formed on the central portion of the mounting base 13. The working coil 11 which is wound in the form of spiral is installed on the ferrite cores 12. A pair of power source terminals 14 for being supplied with a power source are formed on the both ends of the working coil 11.
FIG. 4 is an enlarged view of the part A of the working coil 11 shown in FIG. 2. Referring to FIG. 4, the working coil 11 having a plurality of fine lines 110 is shown. The reason for forming the working coil 11 with the plurality of fine lines 110 is to reduce a loss by a conductor skin depth effect; ##EQU1## according to the high speed switching of the working coil. Here, L.sub.s is a penetration depth. The fine lines 110 have a small diameter of approximately 0.2 to 0.4 mm, and are coated with enamel. The fine lines 110 are formed of a conductive material having a small D.C. resistance in order to minimize the loss due to the generation of joule heat and maximize the generation efficiency of high frequency AC magnetic field.
As described above, in the conventional heating apparatus comprising a working coil composed of a plurality of fine lines, there are problems in that the magnetic field distribution is not uniform and the change of inductance of the working coil is difficult. Specifically, the conventional heating apparatus has a problem in that the distribution of the generated magnetic field is not uniform due to difference in the length winding rates of the fine lines. Also, when the distribution of the winding working coil is changed to vary the inductance of the heating apparatus, the change in the metal mold is also followed, thereby increasing the production cost.